If you don’t know about smart keywords, basically you can set phrases as shortcuts for specific bookmarks. I want a folder with all of my keywords synced, and that folder only. On each profile or account I have have whatever other folders and the placement of the folder with the keywords doesn’t matter as long as they have the same name.
After a related frustration with firefox recently my guess is that you can not. At least not in a default way.
Tags and keywords are saved in mozillas proprietary jsonlz4 format which is a one way road. Other bookmark exports do not include this metadata and the only way to import jsonlz4 will overwrite the existing bookmarks.
I really hope some one corrects me, if not consider making a profile (firefox -p or about:profiles)and sync the profile.